Hypomorphic temperature-sensitive alleles of NSDHL cause CK syndrome

CK syndrome (CKS) is an X-linked recessive intellectual disability syndrome characterized by dysmorphism, cortical brain malformations, and an asthenic build. Through an X chromosome single-nucleotide variant scan in the first reported family, we identified linkage to a 5 Mb region on Xq28. Sequencing of this region detected a segregating 3 bp deletion (c.696_698del [p.Lys232del]) in exon 7 of NAD(P) dependent steroid dehydrogenase-like (NSDHL), a gene that encodes an enzyme in the cholesterol biosynthesis pathway. We also found that males with intellectual disability in another reported family with an NSDHL mutation (c.1098 dup [p.Arg367SerfsX33]) have CKS. These two mutations, which alter protein folding, show temperature-sensitive protein stability and complementation in Erg26-deficient yeast. As described for the allelic disorder CHILD syndrome, cells and cerebrospinal fluid from CKS patients have increased methyl sterol levels. We hypothesize that methyl sterol accumulation, not only cholesterol deficiency, causes CKS, given that cerebrospinal fluid cholesterol, plasma cholesterol, and plasma 24S-hydroxycholesterol levels are normal in males with CKS. In summary, CKS expands the spectrum of cholesterol-related disorders and insight into the role of cholesterol in human development.

Severe mental retardation, seizures, and hypotonia due to deletions of MEF2C

We present four patients, in whom we identified overlapping deletions in 5q14.3 involving MEF2C using a clinical oligonucleotide array comparative genomic hybridization (CGH) chromosomal microarray analysis (CMA). In case 1, CMA revealed an approximately 140 kb deletion encompassing the first three exons of MEF2C in a 3-year-old patient with severe psychomotor retardation, periodic tremor, and an abnormal motor pattern with mirror movement of the upper limbs observed during infancy, hypotonia, abnormal EEG, epilepsy, absence of speech, autistic behavior, bruxism, and mild dysmorphic features. MRI of the brain showed mild thinning of the corpus callosum and delay of white matter myelination in the occipital lobes. In case 2, an approximately 1.8 Mb deletion of TMEM161B and MEF2C was found in a child with severe developmental delay, hypotonia, and seizures. Patient 3 had epilepsy, hypotonia, thinning of the corpus callosum, and developmental delay associated with a de novo approximately 2.4 Mb deletion in 5q14.3 including MEF2C and five other genes. In case 4, a de novo approximately 5.7 Mb deletion of MEF2C and five other genes was found in a child with truncal hypotonia, intractable seizures, profound developmental delay, and shortening of the corpus callosum on brain MRI. These deletions further support that haploinsufficiency of MEF2C is responsible for severe mental retardation, seizures, and hypotonia. Our results, in combination with previous reports, imply that exon-targeted oligo array CGH, which is more efficient in identifying exonic copy number variants, should improve the detection of clinically significant deletions and duplications over arrays with probes spaced evenly throughout the genome.

Phenotypic characterization of Bbs4 null mice reveals age-dependent penetrance and variable expressivity

Bardet-Biedl syndrome (BBS) is a rare oligogenic disorder exhibiting both clinical and genetic heterogeneity. Although the BBS phenotype is variable both between and within families, the syndrome is characterized by the hallmarks of developmental and learning difficulties, post-axial polydactylia, obesity, hypogenitalism, renal abnormalities, retinal dystrophy, and several less frequently observed features. Eleven genes mutated in BBS patients have been identified, and more are expected to exist, since about 20-30% of all families cannot be explained by the known loci. To investigate the etiopathogenesis of BBS, we created a mouse null for one of the murine homologues, Bbs4, to assess the contribution of one gene to the pleiotropic murine Bbs phenotype. Bbs4 null mice, although initially runted compared to their littermates, ultimately become obese in a gender-dependent manner, females earlier and with more severity than males. Blood chemistry tests indicated abnormal lipid profiles, signs of liver dysfunction, and elevated insulin and leptin levels reminiscent of metabolic syndrome. As in patients with BBS, we found age-dependent retinal dystrophy. Behavioral assessment revealed that mutant mice displayed more anxiety-related responses and reduced social dominance. We noted the rare occurrence of birth defects, including neural tube defects and hydrometrocolpos, in the null mice. Evaluations of these null mice have uncovered phenotypic features with age-dependent penetrance and variable expressivity, partially recapitulating the human BBS phenotype.

Of mice and men: tyrosinase modification of congenital glaucoma in mice but not in humans

PURPOSE

Primary congenital glaucoma (PCG) is an autosomal recessive ocular trait caused by mutations in the gene for cytochrome P4501B1 (CYP1B1). Although PCG is often considered to be fully penetrant, the disease shows 50% penetrance in some Saudi Arabian families. The familial segregation of the nonpenetrance suggests a genetic modifier. Recently, tyrosinase (Tyr) deficiency was found to worsen the drainage structure/ocular dysgenesis phenotype of Cyp1b1-/- mice, suggesting that Tyr is a modifier of the phenotype. In the current study, tyrosinase (TYR) was investigated in human PCG.

METHODS

A genome-wide screen, a single nucleotide polymorphism (SNP) analysis in the TYR chromosomal region 11q13-q21, and sequencing of the TYR gene was performed with individuals from Saudi Arabian families with multiple, clinically confirmed, molecularly proven, nonpenetrant members.

RESULTS

The study outcome did not support TYR as a modifier of the PCG phenotype in this population. The sequencing data showed no TYR mutations in the nonpenetrant family members and no difference in polymorphism frequencies between nonpenetrant or fully penetrant families.

CONCLUSIONS

TYR is not a modifier of the CYP1B1-associated PCG phenotype in the Saudi Arabian population.

Heterozygous and homozygous mutations in PITX3 in a large Lebanese family with posterior polar cataracts and neurodevelopmental abnormalities

PURPOSE

The PITX3 gene, which codes for a homeobox bicoidlike transcription factor is responsible for dominant cataract and anterior segment mesenchymal dysgenesis in humans. In the current study, a family with autosomal dominant posterior polar cataract (PPC) and a PITX3 mutation that cosegregates with the disease was examined. Also studied were two siblings who were homozygous for the PITX3 mutation who had microphthalmia and significant neurologic impairment.

METHODS

A genome-wide screen, linkage analysis in the PITX3 chromosomal region 10q25, haplotype analysis, and sequencing of the PITX3 gene were performed on 28 affected and 14 unaffected member of a three-generation Lebanese family.

RESULTS

Genome-wide linkage analysis showed a lod score of 3.56 at theta = 0.00 on chromosome 10 at area q25. Analysis of the haplotypes and phenotypes confined the disease locus to a region on 10q25 between the markers D10S1239 and D10S1268. A candidate gene, PITX3, maps to that region. Sequencing of the PITX3 gene revealed a heterozygous G deletion mutation in 25 of the 42 family members. In addition, two siblings from a consanguineous marriage were found to be homozygous for the deletion.

CONCLUSIONS

This is the first report of homozygous PITX3 mutations in humans. The phenotype in these individuals highlights the role of PITX3 in ocular and central nervous system (CNS) development.

Recurrent biparental hydatidiform mole: additional evidence for a 1.1-Mb locus in 19q13.4 and candidate gene analysis

OBJECTIVES

A maternal autosomal recessive mutation causing recurrent biparentally inherited complete hydatidiform moles (BiCHM) in affected women was previously mapped to a 12.4-cM interval in 19q13.4, which was recently further narrowed to a smaller 1.1-Mb region at the centromeric end. It is believed that the mutant gene in this condition is a major contributor to the regulation of imprinting in the maternal germline. To confirm and possibly narrow the critical interval we studied additional rare familial and recurrent cases.

METHODS

Using polymorphic marker analysis, we first confirmed biparental inheritance on the studied molar tissues. We then performed targeted homozygosity mapping with markers in 19q13.4 on DNA from affected women of a new large consanguineous pedigree, an additional potentially familial case, and three cases with sporadic recurrent CHM. Direct sequencing of coding exons and Southern analysis with a coding-region probe for one candidate gene (NALP5) was also performed.

RESULTS

Biparental inheritance was confirmed for those molar tissues available for analysis. All women, except for one of the isolated cases, were homozygous for markers in the identified 1.1-Mb region in 19q13.4. No mutations or large genomic rearrangements were found in NALP5 (MATER), a gene with oocyte-specific expression. Heterozygosity for a single-nucleotide polymorphism in exon 13 of NALP5 in one patient may refine the candidate region to 1.0 Mb.

CONCLUSIONS

The reported candidate region for BiCHM in 19q13.4 was confirmed in additional families, further establishing it as the major locus that harbors a gene mutated in this condition.

Retinal degeneration associated with RDH12 mutations results from decreased 11-cis retinal synthesis due to disruption of the visual cycle

Retinoid dehydrogenases/reductases catalyze key oxidation-reduction reactions in the visual cycle that converts vitamin A to 11-cis retinal, the chromophore of the rod and cone photoreceptors. It has recently been shown that mutations in RDH12, encoding a retinol dehydrogenase, result in severe and early-onset autosomal recessive retinal dystrophy (arRD). In a cohort of 1011 individuals diagnosed with arRD, we have now identified 20 different disease-associated RDH12 mutations, of which 16 are novel, in a total of 22 individuals (2.2%). Haplotype analysis suggested a founder mutation for each of the three common mutations: p.L99I, p.T155I and c.806_810delCCCTG. Patients typically presented with early disease that affected the function of both rods and cones and progressed to legal blindness in early adulthood. Eleven of the missense variants identified in our study exhibited profound loss of catalytic activity when expressed in transiently transfected COS-7 cells and assayed for ability to convert all-trans retinal to all-trans retinol. Loss-of-function appeared to result from decreased protein stability, as expression levels were significantly reduced. For the p.T49M variant, differing activity profiles were associated with each of the alleles of the common p.R161Q RDH12 polymorphism, suggesting that genetic background may act as a modifier of mutation effect. A locus (LCA3) for Leber congenital amaurosis, a severe, early-onset form of arRD, maps close to RDH12 on chromosome 14q24. Haplotype analysis in the family in which LCA3 was mapped excluded RDH12 as the LCA3 gene and thus suggests the presence of a novel arRD gene in this region.

Characterization of a new syndrome that associates craniosynostosis, delayed fontanel closure, parietal foramina, imperforate anus, and skin eruption: CDAGS

We describe the clinical characterization, molecular analyses, and genetic mapping of a distinct genetic condition characterized by craniosynostosis, delayed closure of the fontanel, cranial defects, clavicular hypoplasia, anal and genitourinary malformations, and skin eruption. We have identified seven patients with this phenotype in four families from different geographic regions and ethnic backgrounds. This is an autosomal recessive condition that brings together apparently opposing pathophysiologic and developmental processes, including accelerated suture closure and delayed ossification. Selected candidate genes--including RUNX2, CBFB, MSX2, ALX4, TWIST1, and RECQL4--were screened for mutations, by direct sequencing of their coding regions, and for microdeletions, by fluorescent in situ hybridization. No mutations or microdeletions were detected in any of the genes analyzed. A genomewide screen yielded the maximum estimated LOD score of +2.38 for markers D22S283 and D22S274 on chromosome 22q12-q13. We hypothesize that the gene defect in this condition causes novel context-dependent dysregulation of multiple signaling pathways, including RUNX2, during osteoblast differentiation and craniofacial morphogenesis.

A mixed epigenetic/genetic model for oligogenic inheritance of autism with a limited role for UBE3A

The genetic contribution to autism is often attributed to the combined effects of many loci (ten or more). This conclusion is based in part on the much lower concordance for dizygotic (DZ) than for monozygotic (MZ) twins, and is consistent with the failure to find strong evidence for linkage in genome-wide studies. We propose that the twin data are compatible with oligogenic inheritance combined with a major, genetic or epigenetic, de novo component to the etiology. Based on evidence that maternal but not paternal duplications of chromosome 15q cause autism, we attempted to test the hypothesis that autism involves oligogenic inheritance (two or more loci) and that the Angelman gene (UBE3A), which encodes the E6-AP ubiquitin ligase, is one of the contributing genes. A search for epigenetic abnormalities led to the discovery of a tissue-specific differentially methylated region (DMR) downstream of the UBE3A coding exons, but the region was not abnormal in autism lymphoblasts or brain samples. Based on evidence for allele sharing in 15q among sib-pairs, abnormal DNA methylation at the 5'-CpG island of UBE3A in one of 17 autism brains, and decreased E6-AP protein in some autism brains, we propose a mixed epigenetic and genetic model for autism with both de novo and inherited contributions. The role of UBE3A may be quantitatively modest, but interacting proteins such as those ubiquitinated by UBE3A may be candidates for a larger role in an oligogenic model. A mixed epigenetic and genetic and mixed de novo and inherited (MEGDI) model could be relevant to other "complex disease traits".

The fibroblast growth factor receptor-4 Arg388 allele is associated with prostate cancer initiation and progression

PURPOSE

Increased expression of fibroblast growth factors that can activate the fibroblast growth factor receptor-4 (FGFR-4) occurs in a substantial fraction of human prostate cancers in vivo. A germline polymorphism of the FGFR-4 gene resulting in expression of arginine at codon 388 (Arg388) is associated with aggressive disease in patients with breast and colon cancer. We therefore sought to determine whether the FGFR-4 Arg388 allele was associated with prostate cancer incidence and/or the occurrence of aggressive disease.

EXPERIMENTAL DESIGN

The FGFR-4 genotype of men undergoing radical prostatectomy and controls of the same race was determined and the genotype correlated with clinical and pathologic markers of disease aggressiveness. PNT1A cell lines expressing predominantly the FGFR-4 Arg388 or Gly388 allele were established, and cell migration and invasiveness of these cells were assessed by a wounding assay and by quantitative determination of invasion through Matrigel. Expression of urokinase-type plasminogen activator receptor was determined by quantitative RT-PCR and enzyme-linked immunoabsorption assay.

RESULTS

Homozygosity for the FGFR-4 Arg388 allele is strongly associated with the occurrence of prostate cancer in white men. The presence of the FGFR-4 Arg388 allele is also correlated with the occurrence of pelvic lymph node metastasis and biochemical (prostate-specific antigen) recurrence. Expression of FGFR-4 Arg388 in immortalized prostatic epithelial cells results in increased cell motility and invasion through Matrigel and was associated with increased expression of urokinase-type plasminogen activator receptor.

CONCLUSION

The FGFR-4 Arg388 allele is associated with both an increased incidence and clinical aggressiveness of prostate cancer and results in changes in cellular motility and invasiveness in immortalized prostate epithelial cells consistent with the promotion of metastasis.

Congenital polycythemias/erythrocytoses

Congenital polycythemias may result from inherited defects in hypoxia sensing, from inherited intrinsic defects in red blood cell precursors, or from inherited conditions that cause low tissue oxygen tension and secondary polycythemia. Conditions of defective hypoxia sensing feature inappropriately normal or elevated serum erythropoietin (Epo) concentrations in the setting of normoxia and erythrocytosis. They are often due to homozygous or compound heterozygous germline mutations in the von Hippel Lindau tumor suppressor gene (VHL) but without increased incidence of tumors. Affected persons have a high risk of arterial thrombosis and early mortality. The molecular biology of rare polycythemic patients with a single mutated VHL allele remains obscure. Primary congenital and familial polycythemias are characterized by low Epo levels and increased erythroid precursor responsiveness to Epo. They are often due to heterozygous gain-of function mutations in the gene for erythropoietin receptor (EPOR). Secondary congenital polycythemias have low tissue oxygen tension due to hemoglobins with high affinity for oxygen, low erythrocyte 2,3 biphosphoglycerate levels, methemoglobinemia or cyanotic heart or lung disease. Whether phlebotomy therapy reduces complications and prolongs survival in congenital polycythemia is not known.

Genetic association analysis of chronic mountain sickness in an Andean high-altitude population

BACKGROUND AND OBJECTIVES

Millions of people live above an altitude of 2,500 m and are at risk of chronic mountain sickness (CMS), a disorder of excessive red cell and hemoglobin production. Preferential ethnic backgrounds, familial character, and heritability studies have suggested that genetic factors make a major contribution to the pathogenesis of CMS, thus our goals were to exploit a probable founder or population admixture effect in the Andean population to determine the genetic determinants of the extreme erythropoietic responses and CMS.

DESIGN AND METHODS

The association of functional candidate genes with severe polycythemia was studied in Andean subjects from Cerro de Pasco, Peru (altitude 4,438 m). We used microsatellites linked to candidate genes known to be involved in hypoxia sensing and erythropoiesis: erythropoietin, erythropoietin-receptor, hypoxia-inducible factor-1a, von Hippel-Lindau, prolyl hydroxylase domain containing 1, 2, 3, and phosphatase and tensin homolog deleted on chromosome ten. Analysis of co-variance (ANCOVA) was used to test the effect of genotypes on hemoglobin values.

RESULTS

Case-control comparisons revealed no significant difference in genotype and allele frequencies at any marker. Initial analysis, with age as a covariate, showed a possible association between PHD3 (marker D14S1049) and severe polycythemia (p=0.05). After the inclusion of alternative co-variates and after adjusting for multiple comparisons, no p values could be considered statistically significant.

INTERPRETATION AND CONCLUSIONS

Our study does not find evidence of associations between the polymorphisms linked to the candidate genes and severe polycythemia; this does not, however, exclude that variations in these genes contribute to polycythemia and possibly CMS.

A novel mouse model of hypogonadotrophic hypogonadism: N-ethyl-N-nitrosourea-induced gonadotropin-releasing hormone receptor gene mutation

An autosomal-recessive mutation that causes hypogonadotrophic hypogonadism was isolated during an N-ethyl-N-nitrosourea mutagenesis screen in mice. Affected males had micropenis and small, undescended testes with spermatogenesis arrested at the pachytene stage of meiosis, leading to sterility. Androgen-sensitive organs were small and immature. Affected females were externally normal but sterile with small ovaries due to an arrest at the secondary stage of folliculogenesis, and the uterus and oviducts were thin and immature. Circulating reproductive hormones were significantly decreased in affected males and females. There was also a dramatic reduction in the numbers of FSH- and LH-producing gonadotrophs. Meiotic mapping of the mutation and candidate gene sequencing determined that the N-ethyl-N-nitrosourea-induced lesion is in the third transmembrane domain of the GnRH receptor gene (Gnrhr). In vitro studies indicate that the mutant receptor is not coupled to the plasma membrane signal transduction system. Moreover, this mutant cannot be rescued with defined GnRH receptor pharmacoperones (pharmacological chaperones), an approach that rescues many other misfolded mutants. The mutant GnRH receptor was also shown to exert a dominant-negative effect on wild-type receptor function, indicating that the mutant receptor is unable to fold properly and likely misrouted within the cell, not reaching the plasma membrane. Surprisingly, Gnrhr mutant transcripts were significantly up-regulated in the pituitaries of Gnrhr mutants, revealing a previously unknown autoregulatory feedback loop. This is the first report of a mouse with a Gnrhr loss of function mutation. These GnRH-insensitive mice provide a novel animal model for the study of human idiopathic hypogonadotrophic hypogonadism.

Expanding the phenotype of alveolar capillary dysplasia (ACD)

OBJECTIVES

To define the phenotype of congenital alveolar capillary dysplasia (ACD) as a first step toward mapping the responsible gene(s).

STUDY DESIGN

Analysis of pathology reports and microscopic slides of 23 subjects with ACD and sequence analysis of two candidate genes.

RESULTS

Our review of the pre- and postmortem records delineates both the natural history of this condition and the associated anomalies. Our collection of families corroborates the likely autosomal recessive nature of this condition in some families and provides additional data for genetic and prenatal counseling. Anomalies of many organ systems were detected either in the prenatal period or during the hospital course. However, some major anomalies were not detected until postmortem examination. Left-right asymmetry and gastrointestinal malrotation emerge as important, previously recognized but underappreciated phenotypic features of ACD. Finally, we used sequence analysis to exclude mutations in the coding region of two candidate genes, bone morphogenetic protein type II receptor (BMPR2) and endothelial monocyte-activating polypeptide II (EMAP II), as candidates for ACD.

CONCLUSIONS

Understanding the clinical spectrum of ACD and the cloning of an "ACD gene" both have implications for counseling, for prenatal testing, and for understanding the molecular pathophysiology of ACD and other organ malformations that are associated with this condition.

Loss of Heterozygosity Occurs via Mitotic Recombination in Trp53+/− Mice and Associates with Mammary Tumor Susceptibility of the BALB/c Strain

Loss of heterozygosity (LOH) occurs commonly in cancers causing disruption of tumor suppressor genes and promoting tumor progression. BALB/c-Trp53(+/-) mice are a model of Li-Fraumeni syndrome, exhibiting a high frequency of mammary tumors and other tumor types seen in patients. However, the frequency of mammary tumors and LOH differs among strains of Trp53(+/-) mice, with mammary tumors occurring only on a BALB/c genetic background and showing a high frequency of LOH, whereas Trp53(+/-) mice on a 129/Sv or (C57BL/6 x 129/Sv) mixed background have a very low frequency of mammary tumors and show LOH for Trp53 in only approximately 50% of tumors. We have performed studies on tumors from Trp53(+/-) mice of several genetic backgrounds to examine the mechanism of LOH in BALB/c-Trp53(+/-) mammary tumors. By Southern blotting, 96% (24 of 25) of BALB/c-Trp53(+/-) mammary tumors displayed LOH for Trp53. Karyotype analysis indicated that cells lacking one copy of chromosome 11 were present in all five mammary tumors analyzed but were not always the dominant population. Comparative genomic hybridization analysis of these five tumors indicated either loss or retention of the entire chromosome 11. Thus chromosome loss or deletions within chromosome 11 do not account for the LOH observed by Southern blotting. Simple sequence length polymorphism analysis of (C57BL/6 x BALB/c) F1-Trp53(+/-) mammary tumors showed that LOH occurred over multiple loci and that a combination of maternal and paternal alleles were retained, indicating that mitotic recombination is the most likely mechanism of LOH. Nonmammary tumors of BALB/c mice also showed a high frequency of LOH (22 of 26, 85%) indicating it was not a mammary tumor specific phenomenon but rather a feature of the BALB/c strain. In (C57BL/6 x BALB/c) F1-Trp53(+/-) mice LOH was observed in 93% (13 of 14) of tumors, indicating that the high frequency of LOH was a dominant genetic trait. Thus the high frequency of LOH for Trp53 in BALB/c-Trp53(+/-) mammary tumors occurs via mitotic recombination and is a dominant genetic trait that associates with the occurrence of mammary tumors in (C57BL/6 x BALB/c) F1-Trp53(+/-) mice. These results further implicate double-strand DNA break repair machinery as important contributors to mammary tumorigenesis.

Clonal hematopoiesis in familial polycythemia vera suggests the involvement of multiple mutational events in the early pathogenesis of the disease

Familial clustering of malignancies provides a unique opportunity to identify molecular causes of cancer. Polycythemia vera (PV) is a myeloproliferative disorder due to an unknown somatic stem cell defect that leads to clonal myeloid hyperproliferation. We studied 6 families with PV. The familial predisposition to PV appears to follow an autosomal dominant inheritance pattern with incomplete penetrance. All examined females informative for a transcriptional clonality assay had clonal hematopoiesis. We excluded linkage between PV and a number of previously proposed candidate disease loci (c-mpl, EPOR, 20q, 13q, 5q, 9p). Therefore, mutations at these loci are unlikely primary causes of familial PV. The finding of erythropoietin-independent erythroid progenitors in healthy family members indicated the presence of the PV stem cell clone in their hematopoiesis. This finding, together with clonal hematopoiesis in the affected individuals, supports the hypothesis of multiple genetic defects involved in the early pathogenesis of PV.

The worldwide distribution of the VHL 598C>T mutation indicates a single founding event

The first congenital defect of hypoxia-sensing homozygosity for VHL 598C>T mutation was recently identified in Chuvash polycythemia. Subsequently, we found this mutation in 11 unrelated individuals of diverse ethnic backgrounds. To address the question of whether the VHL 598C>T substitution occurred in a single founder or resulted from recurrent mutational events in human evolution, we performed haplotype analysis of 8 polymorphic markers covering 340 kb spanning the VHL gene on 101 subjects bearing the VHL 598C>T mutation, including 72 homozygotes (61 Chuvash and 11 non-Chuvash) and 29 heterozygotes (11 Chuvash and 18 non-Chuvash), and 447 healthy unrelated individuals from Chuvash and other ethnic groups. The differences in allele frequencies for each of the 8 markers between 447 healthy controls (598C) and 101 subjects bearing the 598T allele (P < 10(-7)) showed strong linkage disequilibrium. Haplotype analysis indicated a founder effect. We conclude that the VHL 598C>T mutation, the most common defect of congenital polycythemia yet found, was spread from a single founder 1,000 to 62,000 years ago.

Possible primary familial and congenital polycythemia locus at 7q22.1-7q22.2

Primary familial and congenital polycythemia (PFCP), inherited as an autosomal dominant trait, has been reported to be associated with mutations in the gene encoding the erythropoietin receptor (EpoR). The clinical features include the presence of isolated erythrocytosis, low erythropoietin (Epo) levels, normal hemoglobin-oxygen dissociation curve, hypersensitivity of erythroid progenitors to exogenous Epo in vitro and no progression to leukemia or myelodysplastic syndrome. Less than 15% of PFCP families have an identifiable EPOR mutation. Abnormalities of other genes are therefore likely responsible for the phenotype of the majority PFCP patients. In this study we report a family segregating PFCP with an autosomal dominant pattern of inheritance, where 7 of 14 members of the family were affected in four generations. This family was studied previously and an EPOR mutation was ruled out by sequencing and by genetic means. Here, we confirmed by linkage analysis that the disease phenotype was not linked to the Epo and EPOR genes. We then performed a genomewide screen with 410 polymorphic markers at average spacing 7.67 cM to locate the chromosomal region responsible for PFCP. We identified a region in 7q22.1-7q22.2 with a suggestive LOD score of 1.84, from our data this is the most likely location of a candidate region responsible for PFCP in this family.

Functional genetic analysis of mouse chromosome 11

Now that the mouse and human genome sequences are complete, biologists need systematic approaches to determine the function of each gene. A powerful way to discover gene function is to determine the consequence of mutations in living organisms. Large-scale production of mouse mutations with the point mutagen N-ethyl-N-nitrosourea (ENU) is a key strategy for analysing the human genome because mouse mutants will reveal functions unique to mammals, and many may model human diseases. To examine genes conserved between human and mouse, we performed a recessive ENU mutagenesis screen that uses a balancer chromosome, inversion chromosome 11 (refs 4, 5). Initially identified in the fruitfly, balancer chromosomes are valuable genetic tools that allow the easy isolation of mutations on selected chromosomes. Here we show the isolation of 230 new recessive mouse mutations, 88 of which are on chromosome 11. This genetic strategy efficiently generates and maps mutations on a single chromosome, even as mutations throughout the genome are discovered. The mutations reveal new defects in haematopoiesis, craniofacial and cardiovascular development, and fertility.

Search for genetic determinants of individual variability of the erythropoietin response to high altitude

There is marked variability in the erythropoietin (Epo) and erythrocytic response to extreme high altitude among mountain dwellers, as well as to hypoxic training among athletes, at least in part because of the variation in the erythropoietic response to hypoxia. We hypothesized that this may be genetically determined. Forty-eight athletes were exposed to 24 h of simulated altitude to 2,800 m in a hypobaric chamber. Serum Epo concentrations were determined at baseline and after 24 h. The Epo responses ranged from -41 to 433% of baseline values after 24 h at simulated altitude. The association of the Epo response to hypoxia with the EPO gene and eight genes involved in Epo regulation utilizing 16 polymorphic dinucleotide repeats was examined. Initial analysis showed a possible association between the EPO gene (marker D7S477) and the increase of the Epo level (P = 0.018). We then tested the possibility that sequence abnormalities in the 3' and 5' hypoxia response elements (3' HRE) and (5' HRE) of the EPO gene could explain the differences in Epo response. We found a 3434 C --> T polymorphism in the 3' HRE sequence. However, this polymorphism showed no correlation with the differences in Epo levels. Further, when we analyzed two additional markers flanking the EPO gene by less than 0.3 cM, we found no association of the allelic variants at these loci with the Epo hypoxic response. In conclusion, we could find not convincing association between markers tightly linked to EPO or eight genes involved in Epo regulation and Epo differential responses to hypoxia.

Toward a genetic etiology of CHARGE syndrome: I. A systematic scan for submicroscopic deletions

CHARGE syndrome is a distinctive subgroup within the more heterogeneous group of patients with CHARGE association. While significant progress has been made in the clinical delineation of this syndrome, the molecular basis of the disorder remains unknown. Based on the complex phenotype, some overlap with DiGeorge/velocardiofacial syndrome (DGS/VCFS), and its estimated population incidence, we hypothesized that CHARGE syndrome could be caused by an unidentified genomic microdeletion. In order to address this hypothesis, we carried out a genome-wide screen for loss of expected heterozygosity using 811 microsatellite markers in ten CHARGE syndrome subjects and their unaffected parents. Eight markers gave results suggestive of failure to inherit one parental allele. These loci were tested with fluorescence in situ hybridization (FISH), but none showed evidence of deletion. This screen sets upper limits on the length of a CHARGE-related microdeletion, should that be the genetic mechanism underlying the phenotype.

Disruption of oxygen homeostasis underlies congenital Chuvash polycythemia

Chuvash polycythemia is an autosomal recessive disorder that is endemic to the mid-Volga River region. We previously mapped the locus associated with Chuvash polycythemia to chromosome 3p25. The gene associated with von Hippel-Lindau syndrome, VHL, maps to this region, and homozygosity with respect to a C-->T missense mutation in VHL, causing an arginine-to-tryptophan change at amino-acid residue 200 (Arg200Trp), was identified in all individuals affected with Chuvash polycythemia. The protein VHL modulates the ubiquitination and subsequent destruction of hypoxia-inducible factor 1, subunit alpha (HIF1alpha). Our data indicate that the Arg200Trp substitution impairs the interaction of VHL with HIF1alpha, reducing the rate of degradation of HIF1alpha and resulting in increased expression of downstream target genes including EPO (encoding erythropoietin), SLC2A1 (also known as GLUT1, encoding solute carrier family 2 (facilitated glucose transporter), member 1), TF (encoding transferrin), TFRC (encoding transferrin receptor (p90, CD71)) and VEGF (encoding vascular endothelial growth factor).

Mutation of TDP1, encoding a topoisomerase I-dependent DNA damage repair enzyme, in spinocerebellar ataxia with axonal neuropathy

Tyrosyl-DNA phosphodiesterase 1 (TDP1) repairs covalently bound topoisomerase I-DNA complexes and is essential for preventing the formation of double-strand breaks that result when stalled topoisomerase I complexes interfere with DNA replication in yeast. Here we show that a deficiency of this DNA repair pathway in humans does not predispose to neoplasia or dysfunctions in rapidly replicating tissues, but instead causes spinocerebellar ataxia with axonal neuropathy (SCAN1) by affecting large, terminally differentiated, non-dividing neuronal cells. Using genome-wide linkage mapping and a positional candidate approach in a Saudi Arabian family affected with autosomal recessive SCAN1, we identified a homozygous mutation in TDP1 (A1478G) that results in the substitution of histidine 493 with an arginine residue. The His493 residue is conserved in TDP1 across species and is located in the active site of the enzyme. Protein modeling predicts that mutation of this amino acid to arginine will disrupt the symmetric structure of the active site. We propose that loss-of-function mutations in TDP1 may cause SCAN1 either by interfering with DNA transcription or by inducing apoptosis in postmitotic neurons.